In the foyer of the London Coliseum, home of English National Opera, a member of the chorus is chatting to a friend. "Come to watch the rehearsal, have you?" he asks. "Come for a bit of pornography, sex, S&M?"

ENO is rehearsing Verdi's A Masked Ball, directed by Calixto Bieito. Not much porn in that, you may think. Think again. Last year the Don Giovanni staged by this Catalan controversialist involved vivid onstage sex and a lot of drug-taking. The critics loathed it; it was dubbed a "crude antimusical farrago", a "coke-fuelled fellatio fest" and "a new nadir in vulgar abuse of a masterpiece". News of first-night booing made the papers and Nicholas Payne, the company's general director, was called upon to justify his decision to hire Bieito.

A Masked Ball is already proving as controversial. Yesterday the papers reported that the chorus are in a "state of rebellion"; that the lead tenor has pulled out; that the dress rehearsal - which would normally be available for ENO Friends to see - has been played behind closed doors. The cast were also said to be unhappy about the opening scene, which involves male singers sitting on toilets, and a scene in which the chorus are called on to give a Nazi salute. There have been other problems, too: Bieito's father is seriously ill and the director has been able to spend only limited time rehearsing in London.

Payne points out that Julian Gavin, the tenor originally lined up to sing the lead role of Gustavus, pulled out last year. "He came to see the production with me in Copenhagen in the summer," he says. "By the end of the performance I could tell he did not want to do it. Gavin is a family man and a practising Catholic. We respected his decision not to sing a rather licentious, lewd character. So we moved John Daszak into the role and gave Julian something next season." For their part the chorus, according to Payne, "have been rehearsing the opera with skill and gusto, and seem very smiley and happy. Unless they are better actors than I had thought."

What does it feel like to be taking over the lead in a production that has hit the headlines even before its opening night? Daszak, who plays the Swedish king who falls in love with his best friend's wife Amelia, is upbeat. "At first I was a bit worried," he says. "Rumours started to reach us about what would happen on stage. People said, 'Do you realise you are going to have to take all your clothes off?' Finally it became clear that things had been blown wildly out of proportion. In fact, I do get down to my boxer shorts. At the moment I'm try ing out different pairs to see which are the least revealing."

None the less, presumably partly in anticipation of trouble, ENO has posted a caveat on its website: "Those who prefer something more traditional should be warned that there will be some violent and adult scenes." These "adult scenes" (not in the original score, naturally) include, apart from the lavatorial opener, male rape, a punishment meted out to a pickpocket who steals Gustavus's wallet while he is consulting Madame Arvidson, the seer. Then there's a nasty bit of domestic violence: Anckarstroem, Amelia's husband, knocks his wife around when he discovers that she has fallen for the king. Madame Arvidson has been transformed into, according to Daszak, "a madam-cum-tarot card reader" and her lair into "a brothel meets Mystic Meg's place". The score indicates that the royal Gustavus visits the seer disguised as a fisherman, or pescator , but Bieito has teased out the slang meaning of the Italian word, which translates loosely as "someone out on the pull". "So I'm dressed as a cross between Elvis and Abba," says Daszak.

Those familiar with the original 18th-century setting of the opera may now be confused. What Bieito has done in his production is to point to mid-1970s Spain - the period of instability following Franco's death and the restoration of the monarchy. The character of Gustavus refers loosely to that of King Juan Carlos.

'The king was quite wild when he was young," explains Daszak. "He would shake off his bodyguards, get on a scooter and go out clubbing and partying. It's about the conflict between duty and pleasure; about the fact that those who have responsibilities may also have outrageous desires. Over the past few days, reading about Princess Margaret's life, I've certainly felt echoes of this. Calixto has taken an essentially melodramatic love story and has tried to find ways of making it believable to us, a modern audience."

To be fair to the critics of Don Giovanni, not all of their quarrels with Bieito's version of the Mozart opera were based on prudery; some objected to the way he had, they said, "forced" his severe, unworkable and even crass interpretation on to the opera. Does Daszak take that criticism of this production of A Masked Ball? "To be honest, I don't take very much notice of reviews," he says. "Opera is an art form that combines so many different elements - music, drama, design - that it's very hard to please everyone. And I don't think it should, either. That's what musicals are for. Opera should be about questioning things, not reinforcing our existing opinions. As far as I am concerned, as long as a director is doing something that he or she believes in, and can convince me, I'm happy.

"It is true that during the first rehearsals I was disappointed, in that at every point the direction seemed to be playing against my natural instincts of the character. This production is certainly not what Verdi intended. But the days when every stage direction in the score was followed precisely are gone."

Daszak, 34, was raised in Manchester, where he studied at Chetham's School of Music and the Royal Northern College of Music. His early prodigious talents as a string player soon stalled. "It was my violin teacher who said I should try singing seriously. She was probably trying to get rid of me."

His mother, he says with characteristic dryness, "loves music but is absolutely tone deaf. Which is probably where I get it from." His father was born in Ukraine and settled in Britain after the second world war. For decades he heard nothing of his family, until he stumbled on his brother's name in a magazine and learned that he had become head of the music college in Lvov, Ukraine, and had taught the celebrated viola player Yuri Bashmet.

Aged 12, Daszak was taken to Verona to see Aida in the Roman amphitheatre. "I think I probably fell asleep in the middle of it," he says. "It was full of tenors who had to be pushed on stage like enormous cars. In a way that is the sort of thing I am fighting against - I'm not the sort of singer who will just stand on stage and sing. But I don't feel like an accomplished actor. I still feel I am learning."

That is unduly modest. Daszak has what composer David Sawer, whose recent opera From Morning to Midnight starred the tenor, calls "the voice of a beautifully flawed hero. He has an abundant theatrical imagination; his acting is alive and present, cool and composed." Daszak would be the first to admit that his voice is not the best in the world. He eloquently describes the juggling acts and compromises that go into the complex art of singing-acting: "the sheer toughness of singing something difficult while doing what you want dramatically". But it is his skilful negotiation of this balance that has lent his recent roles, including Pierre in ENO's recent staging of Prokofiev's epic War and Peace, such vigour and weight. Which is also why he is increasingly in demand by directors such as Richard Jones, who has cast him as Aeneas in Berlioz's The Trojans, the linchpin production of ENO's 2003 schedule.

"My ideal life would be to sing more of the classical-lyrical repertoire in productions that are dramatically interesting," says Daszak. "I'm never going to give an audience a Pavarotti vocal experience. But on the other hand, I'm not going to walk offstage in the middle of a duet to get a glass of iced water. Which is what he did last time I saw him."

· A Masked Ball is in rep at the Coliseum, London WC2 (020-7632 8300), from tonight until April 13.